1. Field of the Invention
The present invention relates to an X-ray imaging tube and a method of manufacturing the same, and more particularly to an X-ray imaging tube having an improved input screen.
2. Description of the Related Art
An X-ray imaging tube is a device which comprises a vacuum envelope having an input end and an output end, an input window closing the input end of the envelope, an input screen located within the envelope and opposing the input window, an anode provided within the output end of the envelope, an output screen located in the output end of the envelope, and beam converging electrodes arranged within the envelope, coaxial with each other, and spaced apart in the axial direction of the envelope 1. The input screen comprises a substrate, a phosphor layer formed on the substrate, and a photoelectric layer formed on the phosphor layer.
In operation, X-rays applied to a subject and passing through it are applied to the input screen through the input window. They pass through the substrate, reaching the phosphor layer. The phosphor layer converts the X-rays into light. The photoelectric layer converts the light into electron beams. The beam-converging electrodes converge the electron beams, and the anode accelerates these electron beams. The electron beams are applied to the phosphor layer of the output screen, which emits rays corresponding to the X-rays, forming an X-ray image of the object. Hence, the X-rays are applied to a visible image. This image is recorded by means of a TV camera, a movie camera, a spot camera, or the like. The X-ray image thus recorded is used for diagnosis.
One of the important characteristics of an X-ray imaging tube of this type is its resolving power, i.e., the ability of producing smallest possible separable images of different points on an object. One of the factors determining the resolution is the quality of the input screen of the X-ray imaging tube.
FIG. 1 is an enlarged view of the input screen of a conventional X-ray imaging tube. As can be seen from FIG. 1, the input screen comprises a substrate 1, an input phosphor layer 2 formed on the substrate 1, and a photoelectric layer 3 formed on the phosphor layer 2. The substrate 1 is made of material having high X-ray transparent, such as aluminum or an aluminum alloy. The input phosphor layer 2 is made of material having high X-ray conversion efficiency, such as cesium iodide activated by sodium (CsI:Na). The photoelectric layer 3 is a multi-layer member made of photoelectric materials such as antimony and alkali metal. As is evident from FIG. 1, the input phosphor layer 2 consists of a number of columnar phosphor crystals 2a.
In the columnar phosphor crystals 2a, X rays 4 applied through the substrate are converted into light beams 5. The light beams 5 propagate in all directions. Those of the beams, which propagate onto circumferential surface of each columnar crystal 2a at incidence angle equal to or greater than 33.degree. C., i.e., the critical angle D of CsI:Na, are reflected totally and, hence, do not degrade the resolution of the X-ray imaging tube. However, those light beams which propagate onto circumferential surface of each crystal 2a at incidence angle less than the critical angle D of CsI:Na propagate into the adjacent columnar crystals 2a, acting as scattering-light therein and inevitably degrading the resolution of the X-ray imaging tube.